This invention relates to oxygen scavenging systems comprising platelet particles having oxygen-scavenging organic cations attached thereto and at least one catalyst to promote the oxygen scavenging. This invention further relates to nanocomposites comprising polymers having the oxygen scavenging systems of the present invention dispersed therein and processes for preparing said nanocomposites.
There are many products, particularly foods and beverages, which are sensitive to oxygen and suffer significant deterioration upon exposure to very low levels of oxygen. To extend the lifetime of oxygen sensitive products such as beer and fruit drinks there are many commercial containers that incorporate oxygen barriers (passive barrier) and/or oxygen absorbers (active barrier). In these designs, an oxygen barrier is used to effectively reduce the permeation of oxygen into the package. For extremely sensitive products an oxygen absorber is used to chemically react with oxygen permeating into the package or oxygen trapped in the headspace during filling. Through careful design, the use of oxygen barrier and/or scavenger materials results in the creation and maintenance of extremely low oxygen levels within the container.
Some of these oxygen sensitive products also require a packaging material that provides a high physical barrier to other gases, such as carbon dioxide for example. An example of such a product is beer, which requires a packaging material that provides a high barrier against egress of carbon dioxide as well as ingress of oxygen.
For many products that require a barrier against oxygen in addition to a barrier against diffusion of another gas, a physical barrier material of a thickness providing an adequate barrier against the gas other than oxygen often does not provide a sufficient barrier against ingress of oxygen.
A polymeric material that is commonly used in packaging applications is polyethylene terephthalate or PET. This material has a number of valuable properties for packaging but lacks sufficient gas barrier for some applications. For example, although PET has adequate oxygen barrier properties for oxygen-insensitive products such as carbonated soft drinks, its oxygen permeability limits its use in packaging for beer, fruit juices and other citrus products, tomato based products and aseptically packed meat. Similarly, while PET provides a sufficient barrier to carbon dioxide for some applications, such as carbonated soft drinks, it does not provide an adequate barrier against carbon dioxide for other applications, such as beer for example.
To improve PET""s gas barrier deficiencies, the concept of a multilayer structure has been taught. Here, polymers that have excellent gas barrier and/or oxygen scavenging properties are combined with PET to produce a layered structure consisting of the individual polymers. Suitable methods for producing multilayer structures include co-injection, co-extrusion, lamination, and coating. Typical polymers suitable as gas barriers include EVOH, PVOH, PVDC and polyamides such as MXD6. Suitable oxygen scavengers include polymers capable of undergoing metal catalyzed oxidation such as MXD6 or polybutadiene, oxidizable metals such as iron, or reduced anthraquinones. Blends of barrier polymers with PET have also been taught as a method to improve the oxygen barrier of packages. Some examples of polymers that have been blended with PET are PEN, EVOH, MXD6, liquid crystal polymers, and BO10. Oxygen absorbers that have been blended into PET include MXD6 with a cobalt catalyst as well as modified polybutadienes incorporated through a reactive extruder.
The formation of a multilayer structures allows the properties of the individual layers to perform independently which allows for optimization of the overall structure. For example, PET can be used to make up the bulk of a container as both the inner and outer layers, while a thin middle layer could consist of EVOH in a three layer bottle. This is advantageous since EVOH, like other barrier or scavenger polymers, is more expensive than PET. Hence, the cost of the structure is minimized by optimizing the amount of barrier or scavenger material needed in the container structure for the application. The ability to form multilayer containers also allows the use of scavenging polymers that by themselves would be precluded due to by-products formed as a result of the scavenging mechanism. By putting a layer of food contact approved polymer, such as PET, between the food and the oxygen-scavenging layer, the migration of by-products into the food is avoided or minimized to acceptable levels.
Although multilayer containers may be less expensive and sturdier than a monolayer container of the barrier or scavenging material itself, the use of different polymers in the layers can result in some problems. For example, the layers sometimes delaminate from each other during use producing a container with diminished structural integrity, barrier properties and clarity. In addition, the use of different polymers in the layers, such as a polyester/polyamide or polyester/polyolefin, increases the difficulty of recycling the material, therefore creating an environmental problem.
The addition of platelet particles, derived from certain clays, to polymers to improve the physical barrier properties of a polymer has also been taught. These so-called nanocomposites are formed by dispersing the platelet particles in the matrix polymer. To effectively improve the gas barrier properties as well as to produce an adequate level of clarity in the polymer, the platelet particles must be partially or fully exfoliated. To achieve this exfoliation, tether molecules are contacted onto the clay. The use of a variety of possible tether molecules has been taught. The tethers are generally organic cations, such as quaternary ammonium salts, phosphonium salts and sulfonium salts. These nanocomposite materials can be used in a monolayer package or in one or more layers of a multilayer structure.
Examples of scavengers incorporated into polyesters are known. For example, WO 98/12127 and WO 98/12244 disclose the preparation of blends of PET containing either oxidizable metals or modified polybutadienes. However, these materials have no passive barrier and are hazy. Also, blends of polyamides with PET are known to be effective scavengers of oxygen in the presence of a cobalt catalyst. However, these blends introduce undesirable contaminants into existing PET recycle streams.
U.S. Pat. No. 5,2736,616 discloses and claims oxygen scavengers containing certain pendant ether moieties.
Thus, there are several oxygen barrier and scavenging technologies known in the art, but none address all the needs for an optimum package.
The present invention relates to novel oxygen scavenging compositions comprising platelet particles having attached thereto at least one tether molecule which scavenges oxygen in the presence of at least one oxygen scavenging catalyst. The present invention further relates to nanocomposites comprising at least one matrix polymer and an oxygen scavenging composition of the present invention dispersed therein.
When the nanocomposites of the present invention are incorporated into a layer of a rigid container or flexible film, the resulting novel articles are suitable for packaging oxygen-sensitive products. The articles containing the composition limit oxygen exposure by acting as an active oxygen barrier and/or a means for scavenging oxygen from within the article.